I routinely house my retic and ball together when space gets short. They cuddle together. I have heard that this is a sign of compatibility. If they stay at opposite ends of the cage it is time to worry. Even if one doesn't kill the other and eat it there could be biting, scarring, etc. I did hear a story about a red tail that killed another red tail that got caught up in the constriction of a prey item. That is why I house together sometimes but always feed seperately.